Despite new technologies that significantly aid in reducing exhaust emissions from internal combustion diesel engines, NOx and particulate emissions are still a subject of environmental concern. Diesel emission standards for vehicle engines are becoming increasingly stringent, and it is difficult to meet governmental emissions regulations by merely relying only on improvements to the diesel engine itself. Thus, diesel engines continue to require some sort of diesel exhaust gas aftertreatment system.
Various exhaust aftertreatment devices have played an essential role in engine emission technologies. Diesel particulate filters (DPF's) are used for particulate matter (PM) control, and lean NOx traps (LNT's) are used for nitrogen oxides (NOx) control. In addition to diesel applications, DPF's and LNT's can be used with lean burn gasoline engines.
Under lean conditions, an LNT adsorbs NOx produced from engine combustion. The adsorption process generally involves two steps. First, engine-out nitric oxide (NO) reacts with oxygen to form nitrogen dioxide (NO2) on an active oxidation catalyst (such as platinum). Second, the NO2 is adsorbed in the form of nitrates by a storage material (such as barium oxide). The LNT may be regenerated under certain conditions, whereby NOx is released and reduced to N2.